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The study was conducted in nine 150-L capacity glass aquaria for 192 h to determine the changes in the microflora in the gut and hepatopancreas of tiger shrimp, Penaeus monodon, after bioaugmenting (water treatment and probiotics). Results showed that luminous bacterial counts were significantly lower in bioaugmented than in nonbioaugmented systems (range 0-5.9 × 10 3 cfu/g and 0-3.2 × 10 3 cfu/g in gut and hepatopancreas, respectively). Biochemical tests of isolates showed that the bioaugmented systems were dominated by Gram-positive Bacillus and Streptococcus and the Gram-negative Vibrio dominated the control. In water-treated and probiotics-fed bioaugmented system, the change in bacterial dominance to Bacillus became evident 2 h in the gut and 24 h in the hepatopancreas. Bacterial dominance shifted to Vibrio species after 120 h. Shrimp in the control were observed to become lethargic after 72 h as Vibrio species significantly increased in number. Dissolved oxygen and unionized ammonia levels were significantly lower in bioaugmented system compared with the control.

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